R. Shikler

POTENTIAL IMAGING OF OPERATING LIGHT-EMITTING DEVICES USING KELVIN FORCE MICROSCOPY

We report on the measurements of two-dimensional potential distribution with nanometer spatial resolution of operating light-emitting diodes. By measuring the contact potential difference between an atomic force microscope tip and the cleaved surface of the light emitting diode, we were able to measure the device potential distribution under different applied external bias. It is shown that the junction built-in voltage at the surface decreases with increasing applied forward bias up to flatband conditions, and then inverted. It is found that the potential distribution is governed by self-absorption of the sub-band-gap diode emission.

Resolution of Kelvin probe force microscopy in ultrahigh vacuum: comparison of experiment and simulation

Abstract An ultrahigh vacuum Kelvin probe force microscope (UHV-KPFM) is used to image the work function change of semiconductor surfaces. We measured the potential drop across the pn-junction on a GaP (1xa01xa00) surface and the potential variation at steps on the GaAs (1xa01xa00) surface and determined the resolution for different tip–sample distances. A simple parallel plate capacitor model is used to simulate the effect of varying tip–sample distance on the detection of the electrostatic forces between tip and sample. The model is applied to a potential step and a potential line. The results for different tip–sample distances are compared to those of the experiment; despite small deviations this simple model describes the experimental situation reasonably well. From the simulations it is concluded that for operation of KPFM in air a serious limitation in resolution has to be accepted.

Kelvin probe force microscopy on III–V semiconductors: the effect of surface defects on the local work function

Abstract The application of Kelvin probe force microscopy (KPFM) in ultra high vacuum (UHV) allows to determine the absolute work function of surfaces with a very high energy (

DIRECT MEASUREMENT OF MINORITY CARRIERS DIFFUSION LENGTH USING KELVIN PROBE FORCE MICROSCOPY

We report on the use of Kelvin force microscopy as a method for measuring very short minority carrier diffusion length in semiconductors. The method is based on measuring the surface photovoltage between the tip of an atomic force microscope and the surface of an illuminated semiconductor junction. The photogenerated carriers diffuse to the junction, and change the contact potential difference between the tip and the sample as a function of the distance from the junction edge. The diffusion length L is then obtained by fitting the measured contact potential difference using the minority carrier continuity equation. The method is applied to measurements of electron diffusion lengths in GaP epilayers.

Two-dimensional surface band structure of operating light emitting devices

We report on measurements of two-dimensional potential distribution with nanometer spatial resolution of operating light emitting diodes. By measuring the contact potential difference between an atomic force microscope tip and the cleaved surface of the light emitting diode, we were able to measure the device surface potential distribution. These measurements enable us to accurately locate the metallurgical junction of the light emitting device, and to measure the dependence of the built-in voltage on applied external bias. As the device is forward biased, the junction built-in voltage decreases up to flat band conditions, and then inverted. It is shown that the potential distribution across the pn junction is governed by self-absorption of the sub-bandgap diode emission.

Scanning probe microscopy of well-defined periodically poled ferroelectric domain structure

We analyze and determine the factors governing the contrast in contact mode atomic force microscopy of domain-structured ferroelectric crystals. The analysis is applied to measurements conducted on KTiOPO4 crystals with artificially created well-defined domain structure. It is found that the amplitude contrast is due to difference in the work functions of the antiparallel domains.

Quantitative evaluation of local charge trapping in dielectric stacked gate structures using Kelvin probe force microscopy

We present a quantitative study of local charge injection into silicon nitride films inside dielectric stack gate structures. The charge is injected using atomic force microscope tips in direct contact with the dielectric layers. The charge distribution is imaged by measuring the contact potential difference between the atomic force microscope tip and the sample surface using Kelvin probe force microscopy. The trapped charge distribution and concentration is calculated using the two-dimensional Poisson equation. It is found that a peak trapped charge density of around 1×1012u2009cm−2 with a spreading of ∼250u2009nm is obtained using 15 V pulses of a few milliseconds in duration.

Microscopic surface photovoltage spectroscopy

We present a microscopic surface photovoltage spectroscopy method. It is based on a tunable illumination system combined with a kelvin probe force microscope, which measures the contact potential difference between a sample surface and a tip of an atomic force microscope. By measuring the contact potential difference as a function of illumination wavelength, the whole surface photovoltage spectrum of a semiconductor sample is obtained with submicrometer spatial resolution. This resolution can be as high as 100 nm, in regions where the minority carrier transport is controlled by drift rather than by diffusion.

Kelvin probe force microscopy using near-field optical tips

Abstract We report on the use of near-field optical force sensors for Kelvin probe force microscopy (KPFM) and surface potential measurements. It is shown that a very good potential sensitivity of less than 5 mV can be obtained using such tips. In addition, it is found that the contact potential difference measured using these tips is independent of the scanning height, as long as it is below 40 nm, and of the applied AC amplitude as long as it is in the range of 1–3 V.

Near-field surface photovoltage

A phenomenon called near-field surface photovoltage is presented. It is based on inducing photovoltage only at a semiconductor space-charge region using near-field illumination. The photovoltage is obtained by measuring the contact potential difference between an optical near-field force sensor and a semiconductor surface under illumination. It is shown that the near-field illumination induces photovoltage at the surface which is principally different from photovoltage induced by far-field illumination. The mechanisms that govern the different far-field and near-field photovoltage response are discussed.